Hurricanes, tornadoes, and other severe wind events are responsible for damaging or destroying thousands of residential and light commercial buildings around the world every year. The wind, during severe winds, flows across a roof causing negative pressures that create a lifting force on the roof structure. This is one of the most common ways that homes or other buildings are destroyed during severe weather. This is due, in part, to inadequate construction methods. It is well known that many homes built in recent decades have roof sheathing that is poorly attached to the supporting wood trusses or rafters because of poor quality installation of nails or staples, with many such nails or staples completely missing their mark and completely failing to fasten these critical roofing elements solidly together. Similarly, roof trusses are oftentimes simply toe nailed into the top of the adjoining walls. However, nails provide little structural advantage during a severe wind event. Traditional homebuilding relies on gravity and friction to keep a roof in place. During severe weather events, however, gravity alone becomes insufficient to prevent the roof from peeling off the rest of the building. Once this occurs, the building is fatally weakened and collapses.
Various products have been developed that attempt to anchor the roof to the walls. In some instances, metal straps are nailed into the wall and wrapped over the trusses. In other instances, temporary straps made from a special low-elongation material have been secured to roof structures and the foundations of homes and other buildings which enhance structural stability. However, hurricane harness strapping requires that the system be manually applied prior to a storm to be effective. This may be impractical where the structure is a vacation home or inhabited by elderly or disabled persons.
A variety of other methods have been developed to increase the structural integrity of buildings so that the buildings are more likely to survive strong winds and storm surges. Considerable attention has been given to strengthening the building codes in such areas as Dade County Florida to mandate constructing new structures with greater resistance to wind loads, and some effort has been put into retrofitting existing structures to better withstand these wind forces. However, the retrofit approaches taken so far are not very good at solving this critical need—either because of poor concepts using adhesives that are likely to fail when buildings are severely stressed in a strong storm event or because otherwise effective concepts (such as spray adhesive/foams) are so expensive, dangerous or cumbersome to install by building owners who might otherwise use them. One aspect of the present technology is aimed specifically at the problem of better securing roof sheathing to rafters and trusses and better sealing the entire roof system against moisture intrusion, and doing so in a way that is less dangerous, less costly, requiring little or no specialized equipment and training, highly effective and reliable, and more readily doable by all contractors and even homeowners themselves.
The Foamseal Hurricane Adhesive, from ITW Devcon Company, is representative of the 2-component spray polyurethane foam products currently being used in the trade. These products are sprayed, while working in the attic, onto the underside of the joints between the 4′×8′ plywood or OSB sheathing and the wood rafters or trusses and onto the underside of the built-in ⅛″-¼″ gaps between the sheathing sheets themselves. These initially liquid products immediately begin to cure as they are applied and become a rigid foam, with high tensile strength, in a matter of minutes. When cured, they are strong and establish good adhesion to the wood components they contact. These cured polyurethane foams reportedly increase the uplift strength of roof sheathing in high winds by 2½ to 3 times and help to seal the built-in joints between plywood sheathing to prevent most or all rainwater from entering the structure when the shingles and tar paper are blown off the roof surface, which typically happens in a hurricane. These 2-component, highly-reactive foam products must be applied by specially-trained contractors using very specialized 2-component equipment, at a cost of many thousands of dollars per home or business. The contractors who do this work must wear protective clothing and chemical respirators to prevent being injured from the fumes (especially any isocyanates released from the chemical reaction making the foam) from the spraying process that might otherwise enter their lungs or eyes and contact their skin. Moreover, working in such protective clothing in the confines of a hot attic can be very physically demanding for anyone who does this work. Also, the companies that provide these chemicals and apply them to roofing structures claim that there is no danger from the chemicals. However, exposure to the chemicals and their resulting foam is likely not safe until all of the fumes have fully dissipated. In some instances, it could take days or weeks for the dangerous fumes to dissipate, all the while exposing the residents of the home.
Alternative methods to the spray adhesive/foam systems have been developed that are easier, cheaper, and less dangerous to apply. One such approach to retrofitting existing roofs involves the application of a fillet bead of construction adhesive to the right-angle joint where 4′×8′ sheets of sheathing contact the supporting rafters or trusses. While applying such a fillet bead of adhesive is relatively simple, quick, and inexpensive, the true effectiveness of this technique is highly doubtful, especially after such a fillet bead is stressed during the expansion and contraction of the roofing elements as they heat and cool, each day, over long periods of time. When an adhesive, even an adhesive that is somewhat flexible, is applied in a joint such as the sheathing/rafter right-angle joint, such a joint design, as is technically well-known in the industry, does not properly dissipate the powerful tensile and shear forces at the bond-line between the adhesive and wood to avoid inducing either premature adhesive, cohesive or substrate failure or weakening when normal expansion/contraction repeatedly occurs over many years from everyday cooling and heating. If such adhesive failure or weakening occurs, then when a high-wind event finally does happen, then the failed or weakened adhesive is not capable of providing the uplift resistance needed when needed most.